What Is Coosa Board? Bluewater 26 vs Nautical 15, Weights and Sizes

Coosa board is polyurethane foam reinforced with layers of fiberglass, made in plywood-size sheets, and it goes where marine plywood rots: transoms, stringers, floors, bulkheads, hatch lids and backing pads. The decision that matters is the grade. Bluewater 26 is the structural board, 26 lb per cubic foot, and it is the one that replaces plywood in a transom or a floor. Nautical 15 is the light, economical board for consoles, liners, seat boxes and fill. A 3/4 in 4 x 8 ft sheet of Bluewater 26 weighs about 52 lb against roughly 70 lb for marine plywood, and it never rots, because there is no wood in it.

What Coosa board is made of

Every grade is closed cell polyurethane foam with glass laid through it. The Bluewater grades carry chopped strand mat plus woven roving; the Nautical grades carry continuous strand mat only, which is why a Nautical board is less stiff than a Bluewater board of the same density. The number in the name is the density in pounds per cubic foot. Coosa paints the sheet corners by grade, blue for Bluewater 26 and white for Nautical 15, so offcuts in the shop stay sorted.

Three things follow from the build. It does not rot and takes on very little water, so a weeping bolt hole does not turn into a black transom. It cuts, drills and sands like a dense wood panel, with carbide tooling. And it is a board, not a flexible core: it stays flat, so it will not follow a crowned deck or a curved hull side the way a scored foam core does.

Coosa board grades: Bluewater 26 vs Nautical 15

Coosa also makes a lighter Bluewater 20 for semi structural panels and a denser Bluewater 32 for heavy screw-holding work. For a repair shop, 26 covers the structural jobs and 15 covers the rest, so those are the two we carry: Coosa Bluewater 26 Composite Board and Coosa Nautical 15 Composite Board.

Coosa board vs marine plywood

Plywood is still stiffer per inch and a fraction of the price, and a plywood transom sealed properly in epoxy lasts a long time. The case for Coosa is the hole you did not seal: a drain, a transducer screw, a swim platform bracket. On plywood, that hole is where the next rebuild starts. On Coosa, it is a hole. Choose Coosa when the boat is worth keeping and you do not want to open that part of it again; choose plywood when the budget decides. Our guide to fiberglassing over plywood covers the sealing schedule if you go that way.

Which grade and thickness for the job

Measure the old core at a cut edge before you order, and measure the transom at the engine cutout before you demolish it: the motor's clamp bracket wants a finished thickness, skins included, and the engine's rigging guide gives the window. On a small skiff with a transom under 48 in wide and 24 in tall, one 4 x 4 ft half sheet of 3/4 in Bluewater 26 cuts into two 24 x 48 in pieces that bond into a 1-1/2 in core. For curved decks, the Divinycell H Contoured PVC Foam Core is scored into small blocks on a glass scrim so it lies down on the crown. Our Divinycell foam core guide covers contoured vs plain and which thickness.

Cutting, fastening and bonding Coosa

• Cutting: a circular saw or table saw with a carbide blade, and carbide drill bits. Coosa's own tooling advice steers you off jigsaws and reciprocating saws on Bluewater; the glass layers tear the edge and eat the blade. Fit the panel with a 1/8 to 1/4 in (3 to 6 mm) gap all round for the bedding fillet.
• Dust: it is glass and foam dust. A fitted P100 respirator with organic vapor cartridges, nitrile gloves and eye protection, and cut outside if you can.
• Prep: scuff both faces with 80 grit, blow the dust off, and wipe with acetone. Just before bedding, wet the bonding face with a thin coat of unthickened resin so the putty grabs.
• Bedding: epoxy thickened with Aerosil - Cabosil (Fumed Silica) to mayonnaise, about 1 cup of loose silica per cup of mixed epoxy, with a spoonful of 1/32" Milled Fiber for strength in the glue line. Trowel it on, press the board in, and weight or clamp it until squeeze-out shows on every edge. Coosa bonds with polyester and vinyl ester as well, but a repair is a secondary bond to cured laminate, and that is epoxy's job.
• Skins: on a transom or a floor, glass over it with 1708 Biaxial Fiberglass, mat side down, lapping 4 to 6 in onto the hull. Figure about 4 oz of resin per square foot per ply by hand, roughly a quart per square yard; the resin calculator sizes the batch.
• Hardware: drill every bolt hole oversize, fill it with thickened epoxy, let it cure, and redrill to size. That gives the bolt a hard sleeve to bear on, so the board does not crush when you torque it down, and it seals the hole.

For bedding and skinning, the Boat Suppliers Epoxy Resin Kit, Thin wets out 1708 and mixes into putty with silica, and its slow 2:1 hardener gives you the working time a full sheet of bedding takes.

How Coosa board ships

A full 4 x 8 ft sheet is too big for parcel carriers, so it travels by truck freight with a liftgate, and one freight charge covers several sheets. The 4 x 4 ft half sheet of Bluewater 26 goes by UPS in an oversize box, which is one more reason to plan a small transom or a run of backing pads around half sheets.

Frequently asked questions

What You Need For This Job

• Coosa Bluewater 26 Composite Board: the structural board for transoms, stringers, floors and backing pads
• Coosa Nautical 15 Composite Board: the light board for consoles, liners and filler panels
• 1708 Biaxial Fiberglass: the skins over the board and the tabbing to the hull
• Boat Suppliers Epoxy Resin Kit, Thin: bedding putty and laminating resin
• Aerosil - Cabosil (Fumed Silica): thickens the bedding putty and fillets
• 1/32" Milled Fiber: adds strength to the glue line